1. Zeolite and Zeolite Membrane
Zeolites are crystalline microporous aluminosilicates known as “molecular sieves.” The uniform pore structure of a zeolite makes it an ideal material for separation by selective adsorption or molecular sieving. Zeolite membranes are polycrystalline thin films that either stand alone or are supported on strong rigid porous substrates of small mass transport resistance, such as macroporous and mesoporous ceramic, stainless steel, and glass plates, tubes, or hollow fibers. Zeolite membranes are commonly synthesized by hydrothermal treatment of the substrate surfaces in liquid phase aluminosilicate precursors (Si/Al=0˜∞). The precursors can be in the form of a clear solution, sol, or gel depending on the chemical compositions. The crystallization of zeolites and the resultant crystal structure are sensitive to the precursor composition, the use of structure directing agents (SDA), the specific route of precursor preparation, and the synthesis temperature and duration.
Because a zeolite membrane is usually in the form of polycrystalline thin film on a porous substrate, the final zeolite membrane comprises inter-grown crystals with minimized intercrystalline spaces. The intercrystalline spaces are considered to be micro-defects because their sizes are larger than the sizes of zeolitic pores, causing significant decrease in selectivity, especially for separations relying upon the molecular sieving effect.
The chemical and structural stability of a zeolite membrane in high temperature atmospheres containing water vapor, acidic compounds, and other corrosive impurities is one of the very important properties of zeolite membranes. Generally, the thermal stability of zeolite structure increases with increasing Si/Al ratio in the framework. The all-silica MFI zeolite (silicalite) is thus far the most stable zeolite with thermal stability up to 1000° C. in gases containing sulfuric acidic vapors. Zeolites with low Si/Al ratios in the framework are generally not suitable in high temperature moist atmosphere applications because of their long term instability.
2. Membrane Substrates
Zeolite membranes are normally grown on a variety of supports, such as alumina and stainless steel, depending on their applications. The supports can be in the form of disks, tubular shapes, or hollow fibers, providing mechanical strength for zeolite membranes. The thickness of zeolite layers on supports is always a compromise between separation performance and the overall flux, which are two major criteria for determination of the quality of the zeolite membrane quality.
The support used for depositing zeolite membranes can be an available material such as amorphous silica, silicon wafers, glass or glass pre-coated with active silica, steel-wool sintered steel composites, porous ceramics (mullite, zirconia, LiTaO3), porous α- or γ-alumina, or composites thereof. Among these, the porous alumina or stainless steel supports are preferred. The supports themselves can be asymmetric to achieve good strength and low flow resistance. When the substrate is selected, the geometry of the support is important when considering the membrane module. A disc is easier to use than a tube for preparation of zeolite membranes, but a tube has a higher surface area to volume ratio. Zeolite membranes have also been made on ceramic hollow fibers and on Al2O3-coated SiC multi-channel monolith supports.
3. Zeolite Membrane Synthesis
A variety of techniques have been developed for fabrication of good quality zeolite membranes. Sol-gel and chemical vapor infiltration techniques have been used for preparation of zeolite-embedded inorganic matrices. Several zeolite membranes, such as MFI, NaA, FAU, AlPO4, SAPO-34, MOR, and DDR, have been prepared by in-situ crystallization and/or seeded secondary growth methods.
In-situ hydrothermal crystallization is one of the most common methods used to prepare supported zeolite membranes. It usually consists of placing a suitable support in contact with a precursor solution or gel in an autoclave. A zeolite film is then grown on the support under hydrothermal conditions. The in-situ crystallization method has the advantage of simplicity of synthesis process that does not include an extra step for coating the seed layer as needed in the secondary growth approach. Multiple hydrothermal synthesis procedures may be needed to minimize micro-defects in the polycrystalline structure. Moreover, when zeolite membranes are grown directly by in-situ crystallization, the membrane quality is affected by the substrate properties. Substrate materials that lack active nucleation sites may result in poor coverage of membrane layer. Substrate surface defects, such as roughness or pinholes, may propagate through the membrane thickness, which lowers the separation selectivity.
The seeded secondary growth method is also commonly employed in the field due to several unique advantages over the in-situ synthesis route. First, by applying a seed layer, the influence of the base substrate can be eliminated to allow much better reproducibility and control of the final membrane quality. Second, because of the ability of the seed crystals to define the crystal structure of the subsequently grown zeolite film, some zeolite membranes can be obtained from template free precursors. The template-free synthesis not only reduces the consumption of expensive template agents but also avoids the template removal step, which may enlarge the undesirable nanometer scale intercrystalline boundaries.
One of the critical steps during the secondary growth synthesis is the preparation of seeds with uniform nano-scale dimension. Seeding can be done by several methods. These include rubbing the support surface with zeolite crystals, pulsed laser ablation of zeolite powder, and coating with colloidal zeolite particles. Each of these approaches presents distinct advantages and disadvantages. Seeding by rubbing is simple and applicable whenever a zeolite powder is available. However, it cannot be used for seeding internal surfaces of tubes and may be difficult to reproduce and to scale up or automate. Seeding by laser ablation requires expensive instrumentation and is difficult to be applied for large supports and for the interior surfaces of tubular supports. Seeding by colloidal particles seems a more general method. Colloidal zeolite seeds can be deposited on either planar or tubular supports using well-known colloidal particle deposition procedures such as dip-coating.
The dry gel conversion method is also called vapor phase transport. First a dry amorphous gel is formed on the support surface, followed by hydrothermal treatment in the presence of small amounts of water vapor or a mixture vapor consisting of a structure-directing agent and water. By this approach, a high concentration of nutrients is confined to the support surface, thus obviating the need for mass transport towards the support during hydrothermal growth. The thickness of the zeolite layer can be controlled by the thickness of the gel layer. This method has been shown to be successful for planar supports but is quite difficult for tubular supports.
4. Zeolite Membrane Modification
The sizes of the intracrystalline channel/pores and guest molecules are often one of the critical parameters in zeolite diffusion. Adjustment of the size and dimensionality of these channel systems is expected to be able to result in molecules being subjected to different diffusional resistance. Among the pore/channel-size controlling techniques, chemical vapor deposition (CVD) of silica on zeolite is an effective method to control the pore-opening size of zeolite and improve the shape-selective adsorption of mixtures of gases and liquids.
U.S. Pat. No. 6,051,517 discloses a modified zeolite or molecular sieve membrane for separation of materials on a molecular scale. The modified membrane is fabricated to wholly or partially block regions between zeolite crystals to inhibit transfer of larger molecules through the membrane, but without blocking or substantially inhibiting transfer of small molecules through pores in the crystalline structure. The modified membrane has a monomolecular layer deposited on the zeolite surface which has coordinated groups of atoms that include (i) a metal atom bonded to oxygen atoms that are bonded to the zeolite substrate atoms (e.g., silicon atoms) and (ii) either hydroxyl groups bonded to the metal atoms or additional oxygen atoms bonded to the metal atoms.
U.S. Published Patent Application No. 2011/0,247,492 discloses a modified FAU zeolite membrane produced by a seeding/secondary (hydrothermal) growth approach in which a structure directing agent such as tetramethylammonium hydroxide is included in the aqueous crystal-growing composition used for membrane formation.